This invention relates to the processing of coated printing plates, and, more particularly, to the washing of the plates after exposure.
In the mass-production printing of newspapers and magazines, the text and pictures are first reproduced as a pattern on a printing plate. Ink is applied to the printing plate and transferred to a piece of paper. One printing plate is used to make thousands of images on paper, by continually reinking the plate.
Many newspapers use the relief method of printing. The most widely practiced approach to relief printing uses photopolymer printing plates, which are typically thin steel or aluminum plates having a thick coating of a photosensitive polymer on one side. To reproduce a composed page of text and pictures in a form suitable for printing, a photographic negative of the page is prepared. The negative is placed on the coated side of the printing plate, and light is shone through the negative to strike the photosensitive material. Where there are transparent areas of the negative, the photosensitive material is exposed. Where there are opaque areas of the negative, the photosensitive material is not exposed. The light must be of a type that causes the photopolymer to react where exposed, and ultraviolet light is ordinarily used for this purpose.
The exposure to ultraviolet light causes the photopolymer material to polymerize, so that the exposed areas become polymerized and the unexposed areas remain unpolymerized. The unpolymerized areas of photopolymer are washed away with water or solvent, and the plates are dried. The polymerized areas of photopolymer, not removed in the wash, are post-cured with ultraviolet light. The resulting plate has raised or relief areas which pick up ink and transmit it to the paper for printing. The plate is then ready for mounting in the press.
The present invention deals with the processing steps in which the unreacted photopolymer is washed away, the plates are dried, and the remaining photopolymer is post-cured. These three steps are usually performed in a single piece of machinery, termed a "plate processor". Existing types of plate processors have a washing section, a drying section, and a post-curing section, and the plates are moved sequentially through these sections either automatically or semi-automatically.
The plate processor should remove the unexposed (or weakly exposed), soft photopolymer material, even from small recesses on the coated side of the plate. If the soft polymer material remains on the surface, ink can accumulate to cause a progressive deterioration in the quality of the printed pages. The later-printed pages may have blurred regions where ink has unintentionally accumulated as a result of incomplete washing. The plate processor should also permit a high throughput of plates, as a reduction in the processing time results in later final news deadlines and better service to the reader of the newspaper. The plate processor should be convenient to use, and as inexpensive as possible in both investment cost and operating cost.
Several different types of plate processors are available, differing primarily in the manner in which the plate is washed (rather than the manner of drying or post-curing). In one approach, the plate is moved through a washing region while the sprays from a large number of low-pressure spray nozzles are directed against the plate. The quality of the finished plates is good, but a large amount of surface foam is generated that is difficult to suppress. This machine can have a high throughput of plates, but is expensive to operate.
Another plate processor uses reciprocating brushes to scrub away the unreacted photopolymer while the plates are passed through a shallow water bath. Most of the unreacted photopolymer material is removed, but the bristles may not be able to reach into small recesses, which may be smaller than the diameter of a single bristle. Unreacted photopolymer material may consequently be left on the surface of the printing plate, particularly in the small recesses of raised areas. The result is a buildup of ink between these raised areas during printing, and a progressive deterioration in print quality. The scrubbing action also can have the unfortunate side effect of damaging some of the polymerized areas, which desirably should not be affected by the washing, by rubbing away some of the polymerized material.
The existing plate processors therefore produce plates that may have unremoved photopolymer or damage to the relief areas, both of which reduce print quality. In addition, they are large in size, using valuable floor space, and are costly to purchase and to operate.
There therefore exists a need for an improved plate washer that produces high quality plates, is compact, and is inexpensive to purchase and to operate. The present invention fulfills this need, and further provides related advantages.